The Resistivity Logs
The flow of electrical current is governed by Ohm's Law: E = Ir, where E is the potential difference (volts), I is the current (amperes), and r is the resistance (ohms).
The resistance of any material will vary with both its size and shape. The intrinsic property of the material to resist current flow is known as the resistivity and is related to resistance by: where A is the cross-sectional area and L is the length of the resistor. For the same volume of material, long, narrow resistors have higher resistances than short, wide ones. The units of resistivity can be seen from the equation to be ohm-meter2/meter, which is usually contracted to ohm-meter or ohm-m. Conductivity is the reciprocal of resistivity with units of mho/meters. R
The resistivities of sedimentary rocks are determined by the rock component types and their geometry. The common reservoir framework minerals of quartz, calcite and dolomite have resistivities in excess of 100 million ohm-meters, and so can be considered essentially as insulators. The same is true for hydrocarbons in the pore space. The fact that resistivities can be logged in sedimentary rocks is principally due to the conductivities associated with the formation water brine and the cation-exchange capacity of clay minerals within the shales.
At a temperature of 77 degrees Fahrenheit, drinking water ( 200 pprn NaCl ) has a resistivity of 26 ohm-m, for sea water ( 35,000 pprn NaCl ) the figure is
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.